FIR stands for Full Intra Request. It is one of the error resiliency mechanisms in WebRTC.
A receiving end can send a FIR message to the sender to indicate that it needs to refresh its received video.
Video is usually sent not as a compressed set of frames, but rather a compressed set of delta frames, which rely on previous frames to be decoded and displayed. If some of the packets get lost along the way, then it becomes somewhere between hard to impossible to continue decoding the stream without some remedy.
Sending out a FIR message indicates the sender to “reset” his encoder and start with a fresh frame.